Zeolites are used in many industrial applications, such as nonphosphate detergent builders, as an ingredient in thin films used for separations, as catalysts, and for making secondary ordered patterns. Zeolite crystals are typically less than about 0.5 μ, but have been prepared as small as nanometer size. These nano-sized zeolite crystals exhibit special properties such as less diffusion resistance and extra large external surface, which are important for applications of ultra-fast ion-exchangers, catalyst supports, adsorbents, thin films, and coatings.
Preparation of Zeolite A from hydrothermal treatment of amorphous gel was developed several decades ago (Kerr, G. T. J. Phys. Chem., 1966, 70, 1047) and has been used as a universal method in industry. Crystal sizes of zeolites are affected by many factors such as the temperature (time), alkalinity, ionic strength, silica/alumina sources, seeding, stirring, and aging. Adding seeds and using organic cations usually yield smaller crystals. Low temperature aging and vigorous agitation favor the nucleation process. (Kacirek, H.; et al., J. Phys. Chem. 1975, 79, 1589; Valtchev, V. et al. Zeolites 1995, 15, 193; Bodart, P. et al., J. Chim. Phys. 1986, 83, 777; Vaughan, D. E. W., Mat. Res. Soc. Symp. Proc. 1988, 111, 89; Zhu, G. et al., Chem. Mater., 1998, 10, 1483–1486; U.S. Pat. No. 3,639,099; U.S. Pat. No. 3,755,538; U.S. Pat. No. 3,808,326; U.S. Pat. No. 4,164,551).
Several nano-size zeolites have been synthesized in clear homogeneous solutions, in which TEOS or SiO2 sols are added to sodium aluminate alkali solution. B. J. Schoeman et al., (Zeolites 1994, 14, 110) prepared nanosize Zeolite A via this route. S. Mintova et al. (Science, 1999, 283, 958) elucidated the mechanism of the nucleation and crystal growth in the synthesis of Zeolite A. Several stages are observed, including formation of gel particles with a diameter of several nanometers, aggregation of these gel particles, nucleation of Zeolite A inside the amorphous gel aggregates, and growth of tiny Zeolite A crystallites at the expense of the embedded amorphous gel. Clear solution syntheses are desirable for mechanistic studies, however, are not economical for practical applications due to the low yields and long crystallization times. G. Zhu et al. (Proc. Int. Zeolite Conf. (12th), 1999, 3, 1863) reported that Zeolite LTA ranging from macro- to nanometer scale was synthesized in the gel systems of Al2O3—SiO2—Na2O-TEA-H2O by controlling the synthesis conditions and alkali sources. In this system, large single crystals of up to 80 μm were obtained.
Schmidt et al. (Inorg. Chem. 2000, 39, 2279) described a route to synthesize nanosize zeolites called confined space synthesis, involving the crystallization of the zeolite inside the pore system of an inert mesoporous matrix. The size distribution of zeolite crystals was controlled by selecting the proper matrix. Zeolite A with an average size in the range of 25–37 nm was prepared, using mesoporous carbon black as inert matrix and was isolated by controlled combustion of the carbon matrix.
Applicants have discovered a simple method to nano-sized Zeolite A crystals of less than 200 nm by utilizing a combination of temperatures, organic cations, precursor mixtures, agitation, and seeding. As compared to clear solution synthesis, Applicants' method corresponds to much larger loading on a volumetric basis, which is closer to traditional synthesis methods used in industry, and therefore would be easier to scale up. The phase transformation step from the precursor mixture to crystals is optimized, which consists of decreasing the crystallization temperature to room temperature, using TMAOH instead of NaOH, adding seeds with small particle sizes, and using open systems under vigorous stirring instead of static hydrothermal systems.